Squeezing light through nanometer-wide gaps in metals can lead to extreme field enhancements, nonlocal electromagnetic effects, and light-induced electron tunneling. This intriguing regime, however, has not been readily accessible because of the lack of reliable technology to fabricate uniform nanogaps with atomic-scale resolution and high throughput.
Subwavelength confinement of optical energy has been demonstrated using metal particles, holes, slits, gaps, and tips. The greatest degree of confinement is obtained within a nanometer-scale gap between two metal surfaces. Point-like nanometric junctions have been created using aggregates of noble-metal nanoparticles, electromigration, electron-beam (e-beam) lithography, or scanning probes, but these methods are not practicable for use in fabricating devices with arbitrary geometries and Ångstrom-scale dimensions over a large area. Furthermore, the amount of light that can be coupled from free space into these point-like junctions is limited, due to size mismatch with the optical wavelength. Transmission measurements on these point-like junctions are impractical, because of the large background of light that passes by next to the junction. These challenges become even greater at longer wavelengths. In particular, squeezing infrared (IR), terahertz (THz) waves, or microwaves into nanometer-scale gaps would lead to extremely high field enhancements, but exploiting resonances at IR, THz, or microwave frequencies will require the nanogap to be extended over micrometers to centimeter length scales. However, such devices are difficult or impractical to manufacture with current techniques.
Other uses for nanoscale or Ångstrom-scale gap structures exist, such as nanogap capacitors, tunnel junctions, field emitters, electron emitters, visible/infrared/terahertz antennas and rectifiers, field-emission display devices, and the like. The manufacture of such devices and other devices having nanoscale or Ångstrom-scale gaps presents challenges with current manufacturing techniques.